Carbonyl sulfide, denoted COS and otherwise known as carbon oxysulfide or carbon oxide sulfide, is a colorless gas that is slightly soluble in water and explosive in air. COS is frequently found in the effluent of coke ovens of steel mills and in Claus sour gas in refineries. In these systems, COS is usually found as a component of gas mixtures containing hydrogen sulfide, carbon monoxide, hydrogen cyanide, nitrogen, carbonyl disulfide, mercaptans, and/or sulfur dioxide. Some effluents also contain large quantities of carbon dioxide.
Carbonyl sulfide is one of the most difficult sulfur compounds to remove from petroleum refinery streams and the like. It has a low boiling point similar to propane, so it cannot be readily separated therefrom by fractionation. It is relatively stable toward acidic reagents and is only slowly affected by strong alkalies such as caustic soda. Hence, previously published methods for its removal involve complex process designs and operations.
In U.S. Pat. No. 2,434,868, for example, Sample and Miller suggested that gases containing carbonyl sulfide be first treated to remove hydrogen sulfide and mercaptans, and the resulting mixture, contacted with an alkaline solution containing aluminum oxide. It has also been suggested that carbonyl sulfide be removed by oxidation in the presence of activated charcoal impregnated with alkali, treatment with derivatives of alkyl amines, and conversion to insoluble metallic sulfides by contacting with a carrier impregnated with metal compounds in the presence of an alkaline reagent (ibid.). Nishino, et al., suggested that carbonyl sulfide could be removed from a gas that does not contain oxygen by contacting the gas with an activated carbon containing a copper compound and an alkali metal and/or alkline earth metal compound (U.S. Pat. No. 4,556,547). To avoid explosions, these methods are generally limited to the abatement of COS in gases containing little or no oxygen, and several are economically unsatisfactory because of the cost of reagents employed. For the latter reason, heterogeneous catalysts such as molybdenum/aluminum oxide, silicon/aluminum oxide, and zeolites suggested for COS abatement are also often considered unfeasible.
Oakley suggested contacting COS-containing gases with an aqueous inorganic alkali solution at temperatures between 70.degree. and 150.degree. C. (U.S. Pat. No. 2,758,005). Holmes and Kosseim disclosed a recycling system between a stage involving the conversion of carbonyl sulfide to carbon dioxide and hydrogen sulfide in the presence of an aqueous alkaline solution in a hydrolysis zone at about 150.degree. to 250.degree. F., followed by absorption of the products in solutions such as aqueous solutions of alkali metal carbonates (U.S. Pat. No. 5,104,630). To achieve significant COS abatement, temperature control is critical in these methods, as well as in various scrubbing techniques, which limits their usefulness. Wet scrubbing is limited by the low solubility of COS in water and slow hydrolysis reactions.
Al-Ghawas, H.A., et al., studied the absorption of carbonyl sulfide in aqueous methyldiethanolamine (Chem. Eng. Sci. 44: 631-639 (1989)), but subsequently reported that the reagent was not selective for absorbing COS from gas mixtures containing hydrogen sulfide or carbon dioxide (Chem. Eng. Sci. 46: 665-676 (1991)). The kinetic data presented in the papers were later questioned by other investigators, who could not confirm the reported reaction rates and postulated that contaminants were present in the reaction mixtures employed (Kittel, R.J., et al., Ind. Eng. Chem. Res. 31: 1269-1274 (1992)).
It would be desirable to have COS abatement methods that can achieve significant COS reduction by conventional separation techniques. It would also be desirable to have COS abatement methods appropriate for the removal of low levels of COS in the presence of high concentrations of other gases that interfere with or decrease the efficiency of some of the above-described methods.